docs: def save(self, filename):
          """Saves the collection to a file.
  
          Parameters
          ----------
          filename : :obj:`str`
              The file to save the collection to.
  
          Raises
          ------
          ValueError
              If the file extension is not .npy or .npz.
          """
          file_root, file_ext = os.path.splitext(filename)
          if file_ext == '.npy':
              np.save(filename, self._data)
          elif file_ext == '.npz':
              np.savez_compressed(filename, self._data)
          else:
              raise ValueError('Extension %s not supported for point saves.' %(file_ext))